Use this thing when you have a bag on your passenger seat that sets off the "buckle your seat belt light". In my 2010 Ford Fusion if I put my laptop bag on seat the car thinks it is a person and alerts me that their seatbelt isn't on.
This thing turns off that alarm and if you like you can attach a rope to it to hold your bag on the seat.
Tested on my 2010 Ford Fusion, however, I don't know if it will fit in other cars or not. The part I derived it from said theirs fit most of the European models so chances are good it may fit your car.
Note: this thing only fits the front seat belt clasps. The rear seat belts are wider so the design will need to be changed to fit them.
Obviously do not use this in place of a normal seat belt when a person is sitting in the seat!
EDIT: There is now a version for the back seat! Again it fits my 2010 Ford Fusion. This new back seat version fits the front belt latch on a 2008 Dodge Durango. It might fit your car too, let me know.
Front Seat Version Fits:
2010 Ford Fusion
Volvo S80 2002 model
Back Seat Version Fits:
2008 Dodge Durango
2010 Ford Fusion (backseat only)
I printed mine with 1 extra shell .27mm layer height and 5% infill.